7.2 Enolate Chemistry

LEARNING GOALS

After   Chapter 7.2,    you will    be  able    to:

Define  tautomerization

Predict the role    of  an  enolate carbanion   in  a   reaction

Describe    the conditions  that    favor   keto    and enol    forms

Identify    the thermodynamically   favored tautomer    of  an  aldehyde    or  ketone:

Due to the acidity of the α-hydrogen, aldehydes and ketones exist in solution as a mixture of two
isomers: the familiar keto form, and the enol form.

KETO–ENOL TAUTOMERIZATION

The enol form gets its name from the presence of a carbon–carbon double bond (the en–
component) and an alcohol (the –ol component). The two isomers, which differ in the placement of
a proton and the double bond, are called tautomers. The equilibrium between the tautomers lies
far to the keto side, so there will be many more keto isomers in solution. The process of
interconverting from the keto to the enol tautomer, shown in Figure 7.2, is called enolization, or,
more generally, tautomerization. By extension, any aldehyde or ketone with a chiral α-carbon will
rapidly become a racemic mixture as the keto and enol forms interconvert, a phenomenon known
as α-racemization.